Here at Nintil I claimed last year that it is unlikely that there is some new useful fundamental physics coming.

When I've made this point in front of an audience, sometimes I've had to clarify myself a lot. I still think it's all mostly addressed in the post itself -, but the topic can do with some extra clarification. In this post I will also point to what I regard as the only candidates I'm aware of for useful fundamental physics

Useful fundamental physics

Fundamental physics

By fundamental physics I mean things like:

The Standard Model

General Relativity

Quantum Field Theory

Quantum Mechanics

An example that doesn't qualify is topological matter, like topological insulators. While they do have potential applications -I can imagine they can be used to build faster, less power-hungry computers-, they do not contravene existing physics; rather they expand on what we already knew.

Useful physics

This is physics that can be leveraged for engineering purposes. New useful physics would enable, presumably, novel forms of transportation, energy, or computation.

An example of advances in physics that are not useful include the observation of gravitational waves (Nobel Prize in Physics, 2017), the discovery of neutrino oscillations (Nobel Prize in Physics, 2015) 1, the discovery of the Higgs Boson (Nobel Prize in Physics, 2013), or more recently, the imaging of a black hole by the Event Horizon Telescope collaboration.

New useful fundamental physics

By "new" I mean something that is not obviously contained or deduced from existing physics, or that contradicts it altogether. This can be either extending an existing model, or completely arguing that, say, General Relativity is wrong and needs a rethink from the ground up, that we would have to bring back absolute space, or whatever.

If there is new useful fundamental physics, we should be able to detect it by experiment, and we ideally we should be able to detect it with current, or near-future technology. Of course, maybe there is useful fundamental physics that will have to wait millenia to be tested and put to use! Here I am concerned about the near future (Up to a century from now).

Because of the epistemic nature of fundamental physics, any proposed drastic revision to them is seen as wrongheaded at best, and pseudoscientific at worst. To avoid the claims of pseudoscience I'm restricting myself, as mentioned before, to experimentally testable claims, but to avoid narrowing the circle too much, I will consider anything vaguely plausible, even if the evidence for it is somewhat negative: If there were solid, definitive, evidence for it, then that would be part of physics already, and it would have been heralded as a revolution. This post from Jess Riedel is a more conservative view on the same kind of question I'm considering here.

An example of a new theory with experimental implications is Roger Penrose's proposed alternative to Quantum Mechanics. His is not merely an interpretation. There are a few interpretations of QM out there, but they all conveniently make the same predictions, for different reasons. Penrose's makes different predictions. This is a case of a tweak to an existing theory, and there is a proposed experiment -FELIX- to test the theory that is within reach of current technology. Alas, it is not clear what the useful application of Penroses'a alternative would be, as far as I know.

Looking for new physics where physics fails

There are phenomena that we currently cannot predict with existing physics, slight deviations from what our best theories tell us. These must be dismissed as data measurement issues, or issues to be solved with what to some feel like hacks.

One such hack is dark matter. There is no direct evidence that dark matter exists; dark matter is something that has been postulated to exist so that observations match theory, but it can also be that the theory is wrong. If there is no extra mass around, then what explains the discrepancies is the fact that we are not accounting for some force, or that existing forces, say gravity, work in a different way.

An example of a theory that has been proposed as an alternative to General Relativity is Modified Newtonian Dynamics - MOND - , wherein the classic equation \(F=ma\) becomes instead \(F=mf(\frac{a}{a_0})a\) where \(f\) is some function and \(a_0\) is some constant. This seems quite ad hoc, and has no first principles grounding -the value of the constant and the kind of function are chosen so that it "works"-, but fits the data; not only it fits the data it was designed to fit, but also other phenomena discovered some time later. However trying to extend the theory to account for relativistic effects has so far been unsuccessful.

Proposed theories of physics that are useful

Quantised Inertia

Readers of this blog will remember Quantized Inertia, a theory proposed by Mike McCulloch from Plymouth University to explain some of the above anomalies. Last year he got some money from DARPAto carry out experiments aiming to validate or disconfirm the theory.

Unlike MOND, there is some physical reasoning behind QI, enabling it to predict the dynamics of galaxy rotation and of a certain kind of start systems (wide binaries) without the need to posit the existence of dark matter.

Theoretical merits of QI aside, it is the only proposed, testable in the near future, theory of fundamental physics that can be put to use, or so McCulloch claims.

If QI is true, then it is possible to build propellentless propulsion systems, meaning that even if they are only capable of producing a tiny acceleration, if that is sustained over time, a spacecraft could reach extremely high speeds. If QI is true and these wild results are true, QI also enables faster than light travel. As noted above, the stronger the contradiction with current physics, the more unlikely a new theory is seen; the fact that the possibility of FTL travel is a consequence of QI may in itself be a reason to reject it.

It's not like QI is troublefree: QI predicts that something like the EMdrive should work, yet recent work (Kößling et al., 2019) with the most accurate instruments so far constructed have found null effects. If your theory predicts X, and X then happens not to be found, your theory is in trouble. Additionally, as tweeted by McCulloch himself - that's some intellectual honesty - the theory as currently formulated has theoretical deficiencies.

Warp drives

A staple of scifi are warp drives: devices that bend spacetime to be able to go from A to B faster than light by altering what "X to Y" mean -the underlying spacetime-.

The only experiment I'm aware of that tries to test if this is actually workable is the White-Juday warp-field interferometer, but so far there has been no results. A paper suggested that the device is incapable of detecting an effect, if there was one.

Conclusion

This clarifies the original post I wrote on the future of fundamental physics from an engineering point of view. I haven't changed my mind relative to last year: the odds that there is no new useful physics is very high, but I have tried to survey the current gamut of proposals that could be true. Hopefully in a few years the experiments discussed above will have been done and then we'll know if there's anything to the underlying theories.

If there are other theories like QI out there, let me know!

Disclaimer

I am no physicist and the only criteria I've used to explore theories that are merely whether they claim useful applications, and whether those claims are testable in the near future.

Acknowledgements

Thanks to Mike McCulloch for reviewing this prior to publication

2

Though one can argue that a few of these have already been explained as measurement errors. The one that is most clear is the galaxy rotation curve anomaly. If one comes to believe there is no dark matter, or no new kind of matter out there, then one has to believe that General Relativity is wrong.

1

I was wrong, as pointed out here, neutrino oscillations do have a potential practical application in monitoring nuclear reactors.